This invention relates to a medical diagnostic ultrasound system and method for contrast agent imaging. In particular, systems and methods for beamforming transmit beams for imaging contrast agents are provided.
Contrast agents are typically imaged using B-mode processes. A plurality of transmit beams focused within a region of interest scan the region of interest. The aperture, apodization, delays and other parameters of the transmit beams interrelate and are optimized for the scan. Information at a fundamental transmit frequency band or a harmonic of the fundamental frequency band (harmonic imaging) is obtained from echo signals responsive to the transmit beams. The amplitude, envelope or power of the information determines the intensity of pixels in the B-mode image.
Contrast agents may be destroyed by the acoustic energy of the transmit beams. Such destruction is undesirable in some circumstances, such as for providing consistent contrast between the contrast agent and surrounding tissue. Where the transmit beams are closely spaced or of a high intensity, contrast agents are more likely destroyed.
The second harmonic response of contrast agents saturates where the transmit beams are of a sufficient energy. Excess energy is backscattered at different harmonics. Furthermore, the second harmonic response of tissue adjacent to contrast agents increases as the acoustic energy increases, reducing the contrast agent-to-tissue specificity.
To avoid destruction and maintain specificity (contrast), the transmit beams for contrast agent imaging are transmitted with less energy. For example, U.S. Pat. Nos. 5,957,845, 5,735,281, 5,694,937, 5,833,613 and 6,110,120, the disclosures of which are incorporated herein by reference, disclose transmitting transmit beams for triggered scans with a standard or increased energy and transmitting other transmit beams between triggers with a decreased or reduced energy. The triggered scans provide desirable information but may destroy contrast agents. Triggering allows contrast agents to reperfuse into the scanned region. The reduced energy transmit beams are used for imaging to maintain the position of the scan plane while minimizing destruction of contrast agents.
U.S. Pat. No. 5,957,845 discloses using xe2x80x9cany technique for altering between two types of frames, one adapted to obtain a high-quality image of tissue containing contrast media and triggered intermittently, and a second adapted not to destroy the bubble imaged by the first frame.xe2x80x9d For example, this patent discloses transmitting at a different frequency for the triggered scans than for the scans between triggers. Using coded excitations to generate the transmit beams may also reduce contrast agent destruction.
Other techniques are used to avoid destruction of contrast agents. U.S. Pat. Nos. 6,005,827 and 6,104,670, the disclosures of which are incorporated herein by reference, disclose focusing techniques to optimize imaging of contrast agents. Compound or line focusing spreads the acoustic energy along a scan line. For spreading the energy along the scan line, a random delay error is superimposed onto focal delays, or an axicon focusing arrangement is used.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiment described below includes a method and system for contrast agent image beamformation. The acoustic energy of transmit beams is spread laterally to reduce contrast agent destruction. For example, a smaller aperture is provided for contrast agent imaging than for a default imaging mode. As another example, an apodization profile with a low amplitude for edge elements for contrast agent imaging is used as compared to the edge elements for a default imaging mode. As yet another example, the focal point used for contrast agent imaging is set to be outside of the region of interest (e.g. a shallow or deep focal point).
Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.